it sounds like you need another table, say ProductElements. Each record would be related to your Products table. Then, you would use that relationship to list the elements/parts associated with each product.

David has put his finger on the key first step: How is your data organized into tables and relationships?

Am I correct that you are trying to link a product's "Bill of Materials"--the list of materials and quantities required to manufacture one unit of product--with the line item records of products linked to an Invoice such that you can get a combined list of all materials needed in order to fulfill that invoice?